The PTH1R binding to scaffolding proteins is key to the actions of parathyroid hormone (PTH) and PTH-related peptide (PTHrP) in a cell- and membrane-specific manner. A receptor for PTH and PTHrP (PTH1R) belongs to G-protein coupling receptors (GPCR). PTH functions primarily to maintain calcium homeostasis through direct action on bone and kidney. PTHrP is a paracrine regulator of cellular growth and differentiation, and its ectopic expression in tumor cells is associated with bone metastasis and hypercalcemia in malignancy. Thus, to study signaling and physiological function of PTH1R via PTH and PTHrP is essential for understanding of mechanisms of diseases and developing novel therapy. The primary research goal is to investigate the role of calpain small subunits (the 28-kDa subunit), a novel interactor, binding to the parathyroid hormone 1 receptor (PTH1R) on PTH and/or PTHrP-mediated signaling and actions. Specific Aim 1 defines calpain-mediated cleavage of the PTH1R and its role on PTH-mediated intracellular events. Specific Aim 2 examines the role of calpain as a modulator of PTH-elicited downstream processes. Specific Aim 3 characterizes the phenotype of mice with osteoblast-specific deletion of Capn4 in vivo and of Capn4-null osteoblsts isolated from these mice.